The invention relates to a low-pressure mercury vapor discharge lamp comprising a discharge vessel with a tubular portion which is transmissive to radiation generated in the discharge vessel,
said discharge vessel enclosing a discharge space provided with a filling of mercury and a rare gas in a gastight manner,
the tubular portion of the discharge vessel being provided with a metal oxide layer and a luminescent layer on a surface facing the discharge space, and
the low-pressure mercury vapor discharge lamp comprising discharge means for maintaining an electric discharge in the discharge vessel.
Mercury constitutes the primary component for (efficiently) generating ultraviolet (UV) light in mercury vapor discharge lamps. A luminescent layer comprising a luminescent material (for example, a fluorescent powder) is present on an inner wall of the discharge vessel for converting UV to other wavelengths, for example, to UV-B and UV-A for tanning purposes (sun panels) or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescence lamps. The discharge vessel of low-pressure mercury vapor discharge lamps is usually circular and has both elongated and compact embodiments. Generally, the tubular discharge vessel of compact fluorescence lamps has a collection of relatively short, straight parts of a relatively small diameter, which straight parts are interconnected by means of bridge parts or via bent parts. Compact fluorescence lamps are usually provided with an (integrated) lamp base. In such embodiments of the low-pressure mercury vapor discharge lamp, the discharge means comprise electrodes which are arranged in the discharge space. An alternative embodiment comprises the electrodeless low-pressure mercury vapor discharge lamps.
A low-pressure mercury vapor discharge lamp of the type described in the opening paragraph is known from U.S. Pat. No. 4,544,997. In the known discharge lamp, the tubular portion of the discharge vessel is provided with a layer of at least an oxide of at least an element of the group of scandium, yttrium, lanthanum, gadolinium, ytterbium and lutetium. The metal oxide layer inhibits attack of the wall of the tubular portion of the discharge vessel due to interaction with mercury and thus has a favorable influence on maintaining the radiation output of the lamp. The metal oxide layer is obtained by rinsing a solution of a metallo-organic compound on the surface of the discharge vessel facing the discharge space and by subsequently drying the film remaining on the surface facing the discharge space and by subsequent sintering.
Due to the metal oxide layer, the mercury consumption of the lamp, i.e. the quantity of mercury which is bound on lamp components during operation of the lamp and is thus no longer available for operation of the lamp, is relatively low as compared with that in lamps which do not have such a metal oxide layer. Nevertheless, a relatively high mercury dosage is necessary for the known lamp so as to realize a sufficiently long lifetime. After the end of the lamp lifetime, injudicious processing is detrimental to the environment.
It is an object of the invention to provide a low-pressure mercury vapor discharge lamp of the type described in the opening paragraph, consuming a relatively small quantity of mercury.
According to the invention, the discharge lamp is therefore characterized in that the luminescent layer comprises an alkali metal oxide.
A number of parts of a low-pressure mercury vapor discharge lamp (for example, the discharge vessel, the luminescent materials, etc.) is not inert to mercury which is present in the discharge. Such parts have the tendency of absorbing mercury. This does not only imply that more mercury should be present in the discharge vessel so as to ensure that the discharge lamp remains in operation during its lifetime, but also that the efficiency of the discharge lamp during its lifetime gradually decreases because many Hg compounds absorb UV and/or visible light. During the lifetime of the low-pressure mercury vapor discharge lamp, the bare glass of the discharge vessel absorbs several milligrams of mercury. By providing a coating (for example, of SiO2) on the discharge vessel, this absorption is reduced by 50%, and by providing a suitable metal oxide layer (for example, a dual coating of SiO2/Al2O3 or SiO2/Y2O3) this absorption is reduced to less than a few hundred xcexcg. The inventors have found that the reduction of the quantity of mercury available for the discharge in the discharge space is mainly caused by the exchange of the alkali metal (for example, Na and/or K) and Hg, and by the absorption of mercury by the surface of the discharge vessel facing the discharge space. During operation of the discharge lamp, mercury enters the wall of the discharge vessel, while the alkali metal oxide simultaneously leaves the wall of the discharge vessel. Mercury consumption through the wall of the discharge vessel is related to imperfections in the metal oxide layer provided on the inner wall of the discharge vessel. Such imperfections give rise to unwanted alkali metal oxide diffusion (for example, diffusion of Na2O and/or K2O) during processing of the discharge lamp, and also to the possibility of mercury atoms adhering to uncoated parts of the discharge vessel, whereafter diffusion of mercury takes place in the glass. Since the diffusion of the alkali metal oxide is generally driven by a concentration gradient between the wall of the discharge vessel and the luminescent layer, the presence of sodium oxide in the luminescent layer causes a much lower diffusion of the alkali metal oxide from the wall of the discharge vessel during processing of the discharge lamp.
By suitably choosing the concentration of sodium oxide in the luminescent layer, the diffusion of the alkali metal oxide from the wall of the discharge vessel can be largely prevented. To this end, a preferred embodiment of the low-pressure mercury vapor discharge lamp according to the invention is characterized in that the alkali metal oxide in the luminescent layer comprises sodium oxide and/or potassium oxide, in which the concentration of alkali metal oxide is 0.001xe2x89xa6Na2Oxe2x89xa60.2% by weight and/or 0.001xe2x89xa6K2Oxe2x89xa60.2% by weight For alkali metal oxide concentrations of less than 0.001% by weight, there is no noticeable reduction of the concentration gradient between the wall of the discharge vessel and the luminescent layer. For alkali metal oxide concentrations of more than 0.2% by weight, the diffusion of Na2O and/or K2O from the wall of the discharge vessel is not further inhibited.
The concentration of the alkali metal oxide in the luminescent layer is preferably 0.002xe2x89xa6Na2Oxe2x89xa60.1% by weight and/or 0.002xe2x89xa6K2Oxe2x89xa60.1% by weight.
An attractive embodiment of the lamp according to the invention is characterized in that the metal oxide layer on the surface of the tubular portion facing the discharge space comprises at least an oxide of at least an element from the group of magnesium, aluminum, titanium, zirconium, and the rare earths. In this description and the claims, the rare earths are understood to be scandium, yttrium, lanthanum and the lanthanides. Such a layer is highly inert so that, also in the long term, the mercury consumption due to reactions of mercury from the filling with the metal oxide layer is small.
Favorable results are obtained with an embodiment of the lamp according to the invention, which is characterized in that the metal oxide layer of the tubular portion comprises aluminum oxide and/or yttrium oxide. Such a layer may be provided, for example, as a suspension of aluminum oxide/yttrium oxide particles, for example, by atomizing the suspension or by causing it to flow across the inner surface of the discharge vessel.
An advantageous embodiment is characterized in that the tubular portion of the discharge vessel has a further metal oxide layer between the surface facing the discharge space and the metal oxide layer (hereinafter also referred to as protective layer). The further metal oxide layer functions as an alkali metal-repellent layer. Such a layer further inhibits the transport of alkali metal ions such as sodium and potassium ions from the wall of the discharge vessel to the discharge space. Mercury consumption by formation of amalgams with alkali metals is thereby further inhibited.
A further favorable embodiment of the lamp according to the invention is characterized in that the further metal oxide layer comprises silicon oxide. Silicon oxide is a very good barrier for alkali metal ions. Such a layer can be easily provided. It is sufficient to rinse a solution of hydrolyzed tetraethyl orthosilicate (TEOS) on the surface of the discharge vessel facing the discharge space. After the silicon oxide thus provided on the surface has been dried, the metal oxide layer can be applied directly. A thermal treatment is favorable so as to enhance the density of the layer. The thermal treatment coincides, for example, with a thermal treatment of the protective layer. If a separate thermal treatment is superfluous for the protective layer, the thermal treatment may coincide with a thermal treatment for removing auxiliary substances such as binding agents from the suspension, if the lamp is provided with a luminescent layer as a suspension of luminescent material.
The discharge vessel has, for example, a luminescent layer which is composed of blue-luminescing barium magnesium aluminate activated by bivalent europium (BAM), green-luminescing cerium gadolinium terbium pentaborate, in which terbium is the activator (CBT), and red-luminescing yttrium oxide activated by trivalent europium (YOX). Alternative luminescent materials are green-luminescing cerium terbium aluminate (CAT), notably suitable for compact fluorescence lamps. Further alternative luminescent materials are blue-luminescing europium strontium halophosphates, for example, europium strontium chlorophosphate (SECA) and green-luminescing cerium terbium lanthanum phosphate (LAP).
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiment(s) described hereinafter.